<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>



    <script>

        // document.write 文档写入   
        // 写入到了body  

        document.write('1                    2');
        document.write('<h1>123</h1>');  // 字符串

        alert('<h1>123</h1>');
        console.log('<h1>123</h1>');

    </script>

    1 2

</body>

</html>